Emission Enhancement and Energy Transfers in YV0.5P0.5O4 Nanoparticles Codoped with Eu3+ and Bi3+ Ions

In this study, solid-state solutions of yttrium orthovanadate-phosphate with varying concentrations of codopants (Eu3+, Bi3+) have been obtained via coprecipitation. An ionic radii mismatch between V5+ and P5+ substituents is manifested in broad XRD lines. The sharpening of the XRD lines is observed with increasing bismuth ions concentration in the Eu3+ codoped YV0.5P0.5O4 matrix. The difference in the number of the Stark components for the 5D0 → 7FJ transitions indicates changes in the lattice and a number of possible Eu3+ sites. A thorough, systematic spectroscopic analysis of YV0.5P0.5O4: x mol % Eu3+, y mol % Bi3+ was conducted at room temperature and 5 K. Metal-to-metal energy transfers occurring between Eu3+, V5+, and Bi3+ optically active ions have been investigated. Additionally, efficiency of the Bi3+-Eu3+ energy transfer (ET) was calculated.


INTRODUCTION
The yttrium orthovanadate and yttrium orthophosphate matrices, doped with europium ions, are popular luminescent phosphors. This is due to their potential applications as laser host materials, polarizers, solar cells, light emitting diodes, host materials for optically active ions, etc. 1−7 YVO 4 and YPO 4 crystallize in the zircon tetragonal system, within the space group I4 1 /amd. 8,9 Hence, a solid-state solution of yttrium orthovanadate-phosphate can be formed. 10 Considering YP 0.5 V 0.5 O 4 , its unit cell is composed of 50 mol % vanadium tetrahedral and 50 mol % phosphate tetrahedral groups, statistically substituted. Furthermore, in this work, yttrium ions in the lattice are statistically substituted with europium and bismuth ions.
In the present work, the fraction of the YPO 4 −YVO 4 − BiVO 4 −BiPO 4 pseudoquaternary diagram (more precisely the shaded area of the YP 0.5 V 0.5 O 4 −BiVO 4 −BiPO 4 pseudo ternary subdiagram) is investigated (Figure 1). The bond valence sums (BVS) were obtained from VESTA, 11 and bond valence parameters were compiled in ref 12. The compounds in the shaded area have a disordered zircon-like crystal structure due to the inner structural characteristics of YPO 4 and YVO 4 . Optically active europium ions are incorporated in YP 0.5 V 0.5 O 4 and (Y,Bi)P 0.5 V 0.5 O 4 for two purposes: to collect information on the local crystal structure and to investigate energy transfer processes involving Bi 3+ . This paper constitutes an extension of previous reports 2,13−16 with more systematic, thorough spectroscopic analysis. Previous works 2,13,14 focus on parame-ters affecting the Eu 3+ emission intensity in micron-sized 14 and nanosized 2,13 systems. It was previously established that codoping YVO 4 :Eu 3+ with P 5+ , Bi 3+ , and Gd 3+ greatly enhances the europium ions' emission intensity. The compositions which maximize Eu 3+ emission intensity are typically Y 0.9 Bi 0.05 Eu 0.05 -P 0.5 V 0.5 O 4 13 or Y 0.45 Gd 0.45 Bi 0.05 Eu 0.05 P 0.5 V 0.5 O 4 . 2 The reasons for this remain obscure.
Trivalent bismuth ion is known as an attractive activator in zircon vanadates. 15,17−23 Furthermore, it has been observed that Bi 3+ is an efficient luminescence sensitizer for trivalent lanthanide ions. 3,24−28 In this study bismuth and europium ions are chosen as codopants, bismuth ions improve the photoluminescence intensity of Eu 3+ . This phenomenon occurs as a result of UV excitation. This is due to the CT transitions from the Bi 3+ 6s energy level to the 5d levels of the vanadate and subsequent energy transfer (ET) to Eu 3+ ions 4f orbitals. 24,29−33 This work emphasizes characterization of the luminescent properties of these two dopants incorporated into YV 0.5 P 0.5 O 4 . This matrix has a disordered structure as a result of phosphate and vanadate units being randomly dispersed throughout the lattice. This occurs because the vanadate units are ∼8% larger than the phosphates. The extent to which this disorder contributes to the efficiency of Bi 3+ -Eu 3+ ET will be investigated in this work. To this end, two series of materials were synthesized. The first one was doped with varying amounts of bismuth ions, while the second was doped with varying amounts of europium ions. The chemical compositions involved doping and codoping YV 0.5 P 0.5 O 4 with xBi 3+ , yEu 3+ where x = 0, 1, 3, 5, 10, 15 mol % and y = 0.5, 1, 2, 5 mol %. The solid state solutions were obtained by the wet chemistry synthesis-coprecipitation method with additional heat-treatment at 800°C for 3 h.
The lanthanide and bismuth oxides were converted into nitrate salts through digestion with an excess of 65% HNO 3 . Thereafter, the formed lanthanide and bismuth nitrates were recrystallized, and the HNO 3 excess was removed. Using deionized water as a solvent, separate aqueous solutions of diammonium phosphate and ammonium metavanadate were made. The vanadium and phosphorus ion sources (NH 4 VO 3 and (NH 4 ) 2 HPO 4 ) were mixed, followed by the nitrates (Y(NO 3 ) 3 , Bi(NO 3 ) 3 , Eu(NO 3 ) 3 ). The liquid mixture was stirred for 1.5 h at approximately 70°C. Aqueous ammonia was used to maintain a pH of 9 during the reaction. The as-prepared precipitates were then washed and centrifuged at least three times, until neutral pH was reached. They were then dried for 24 h at 70°C. The powders were finally crystallized by heat-treatment at 800°C for 3 h in air.
2.2. ICP, XRD, SEM, and TEM Analyses. The crystal structure of synthesized materials was characterized by the X-ray Diffraction (XRD) technique using an X'Pert PRO X-ray diffractometer (Cu Kα1, 1.54060 Å) (PANalytical). Measured XRD patterns were compared to standards of YVO 4 (no. 78074) and YPO 4 (no. 79754) found in the Inorganic Crystal Structure Database (ICSD). Microstructural analyses (particle size, morphology) were performed by electron microscopy. SEM was carried out using an FEI Nova NanoSEM 230. High resolution transmission electron microscopy (HR-TEM) was performed using a Philips CM-20 Super Twin microscope. ICP-OES measurements were conducted on Thermo Scientific ICAP 7000 SERIES.
2.3. Spectroscopic Analysis. The Nicolet iS50 FT-IR from Thermo Scientific was used to collect infrared spectra at 300 K of samples processed in KBr pellets. The room-temperature emission spectra utilized excitation at 397, 340, and 300 nm. These spectra were collected using a FLS1000 photoluminescence spectrometer from Edinburgh Instruments. The same apparatus was used to collect the excitation spectra. The 5 D 0 → 7 F 2 transition at 619 nm was monitored at room temperature for the excitation spectra measurements. Emission spectra were also recorded in response to the 266 nm excitation of a laser diode (CW) at room temperature and detected using the Hamamatsu PMA-12 photonic multichannel analyzer. The emission decay profiles were measured at 300 K using either a Ti:sapphire tunable laser or a Nd:YAG laser, a Hamamatsu R928 photomultiplier, a Jobin-Yvon THR 1000 spectrophotometer, and a digital LeCroy WaveSurfer oscilloscope. Excitation and emission spectra were collected at low temperature (5 K) using a temperature-controlled, continuous-flow liquid helium cryostat: Oxford Model CF 1204. Low temperature excitation spectra were measured with a Dongwoo Optron DM151i monochromator and a 150W ozone free lamp. The low temperature emission spectra were measured using a Dongwoo Optron DM750 monochromator, an Electro-Optical System INC PbS photodiode, or a Hamamatsu R928 photomultiplier. XRD results confirmed the crystal phase purity of YV 0.5 P 0.5 O 4 doped derivatives ( Figure 2). The XRD peaks are broadened and further confirm the structural disorder, as observed earlier. 10,12,20 This broadening originates from the size difference between V 5+ (0.36 Å, C.N. 4) and P 5+ (0.17 Å, C.N. 4), when statistically distributed in YV x P 1−x O 4 solid solution. The lattice strains are influenced by changes in grain size. This is a result of point defects, vacancies, 34 and varying composition 35 as well as dislocations near the grain-boundaries 36 caused by the incompatibility of phosphorus and vanadium atoms. The observed changes in the fwhm of the XRD peaks may indicate the presence of lattice strains. 37 Co-doping with Eu 3+ does not impact the lattice and, by extension, the width of the XRD peaks. In contrast, it is found that codoping with Bi 3+ contributes to narrowing of the XRD peaks ( Figure 2c). This is clearly evidenced by Figure 2b,d. Additionally, increasing the Eu 3+ amount in 10 mol % Bi 3+   By considering Eu 3+ as a local luminescent structural probe, we find three possible environments experienced by Y 3+ ions in   Inorganic Chemistry pubs.acs.org/IC Article the compounds. This is depicted in Figure 3. In YPO 4 , the Eu 3+ ions are surrounded by eight O atoms, thus forming a dodecahedron with D 2d point symmetry. The first cation coordination consists of two P 5+ at 3.01 Å and respectively four P 5+ and four Y 3+ at 3.76 Å in a second coordination. The exact charge (the BVS -Bond Valence Sum) carried by these cations is given in Figure 1. Incorporation of 50% V 5+ creates two additional spheres at 3.14 and 3.89 Å (marked as red in Figure 3) with statistical occupancy. Incorporation of Bi 3+ results in a third sphere (marked as blue in Figure 3) at 4.00 Å. The probability of finding a Bi 3+ ion in this position is P(x) = 1−(1−x) 4 wherein x is the molar percentage of Bi 3+ , e.g., P(0.1) = 35%. The bismuth ions (1.17 Å at C.N. 8) substitute yttrium ions (1.02 Å at C.N. 8) in a statistical manner. Since Bi 3+ is about 15% larger than Y 3+ , its incorporation in the crystal lattice counterbalances (at least partly) the ionic radius mismatch between V 5+ and P 5+ . This inverse relationship between V 5+ −P 5+ and Bi 3+ −Y 3+ ionic radii mismatches reduces the lattice strains in YV 0.5 P 0.5 O 4 and thereby  Inorganic Chemistry pubs.acs.org/IC Article sharpens the XRD lines as the Bi 3+ concentration is raised (Figure 2c). SEM images of YV 0.5 P 0.5 O 4 : 1 mol % Eu 3+ , x mol % Bi 3+ materials are depicted in Figure 4. The powders look micrometric regardless of bismuth ion concentration. As the bismuth content is raised, the particles become smaller, and their surfaces become rougher. EDS maps ( Figure 5) obtained for 1 mol % Eu 3+ , 10 mol % Bi 3+ : YV 0.5 P 0.5 O 4 confirm random distribution of the constituents.
The TEM images in Figure 6 demonstrate that the aggregates consist in fact of nanosized particles. Analysis of the SEM ( Figure 5) and TEM ( Figure 6) images reveals a wide distribution of particle sizes among the samples, although we note that the particles size increases as the Bi 3+ content is raised (i.e., 19, 31, and 62 nm for doping rates of 1, 5, and 15 mol %, respectively). Differences in particles sizes contribute to the narrowing of the XRD peaks in addition to the reordering of the crystal structure. This work sheds a light into a complex structure of Eu 3+ , Bi 3+ : YV 0.5 P 0. 5   Room temperature measurements revealed that all compounds exhibit red emission typical of Eu 3+ upon direct 4f-4f excitation at 397 nm ( Figure 8). This emission increases in intensity with increasing Eu 3+ and Bi 3+ concentrations. The highest observed emission intensity is in samples containing 10 mol % Bi 3+ , 5 mol % Eu 3+ : YV 0.5 P 0.5 O 4 .
The excitation spectra for x mol % Bi 3+ , y mol % Eu 3+ : YV 0.5 P 0.5 O 4 materials for the 5 D 0 → 7 F 2 transition at 619 nm are shown in Figure 9. Three broad transitions are observed at 266, 300, and ≈340 nm (shoulder). They correspond to the O 2− → Eu 3+ , O 2− → V 5+ , and Bi 3+ → V 5+ charge transfers, respectively. 2,13,14,33 The intrinsic 4f-4f excitation lines of Eu 3+ (namely 7 F 0 → 5 L 6 , the 7 F 0 → 5 D 2 , and 7 F 0 → 5 D 1 transitions) are comparatively much less intense. Figure 10 depicts the emission spectra after excitation in the charge transfer bands. These intensities are normalized to the 5 D 0 → 7 F 1 transition of Eu 3+ . In addition to characteristic emission lines of Eu 3+ , broad emission signals are observed. Upon 340 nm excitation, the broad signal represents the emission of the Bi−V metal-to-metal CT, 15 whereas upon 300 nm excitation, the broad signal is more surely due to perturbed vanadate groups. Upon 266 nm excitation, these bandlike emissions possibly overlap. The presence of these emission bands indicates an incomplete sensitization of Eu 3+ luminescence. Two possible sensitization paths are identified. They involve the (VO 4 ) 3units or the Bi−V self-trapped excitons as energy donors and the Eu 3+ ions as energy acceptors. Decay profiles were collected to quantify the efficiency of these energy transfers.
Decay profiles are presented in Figure 11 for 397 nm excitation (inner 7 F 0 → 5 L 6 Eu 3+ transition) or 355 nm excitation (Bi−V MMCT). Corresponding average values of the luminescence lifetimes t av , were calculated as t av = ∫ I(t)t dt/ (∫ I(t) dt). In this case, I(t) represents the emission intensity at time t. These values are provided in Table 2. A plot is proposed in Figure 12 for discussion.
The parameter η in Table 2 is related to the efficiency of the ET from Bi 3+ -to-Eu 3+ in the codoped compounds. It is obtained from the equation where τ doped is the time constant in the presence of the Eu 3+ acceptor, and τ undoped is the time constant without Eu 3+ (i.e., only Bi 3+ ). Figure 12(a) pertains to the inner 7 F 0 → 5 L 6 excitation of Eu 3+ . The left-hand side of the vertical dashed line corresponds to 1 mol % Eu 3+ doped samples with increasing Bi 3+ concentrations (compound A to compound H, excluding compound B, in Table 2). Here, an increase in emission lifetime with increasing bismuth concentration is observed up to 3 mol % Bi 3+ (compound D). These samples (A,C,D,E) also exhibit lower emission intensity relative to samples with higher Bi 3+ concentrations. This suggests a lower radiative probability from the 5 D 0 state in these cases. Raising the Bi 3+ amount further contributes to enhance the Eu 3+ emission intensity, and it correlates with a shortening of the lifetime. This effect has already been observed in YPO 4 :Sm 3+ , Bi 3+ and is ascribed to an increased refractive index of the host lattice. 43 In this case, the radiative probability is increased, contributing to a lower emission lifetime. A synergetic effect with the reordering of the crystal structure is not excluded. This requires further Here, the increase in emission intensity is ascribed to the larger Eu 3+ content that remains beyond the quenching concentration. 44,45 Part (b) of Figure 12 relates to an excitation in the Bi−V MMCT band, the intensity of which exceeds by far that of the 7 F 0 → 5 L 6 transition (Figure 9). In this situation, the Eu 3+ Figure 10. Emission spectra for x mol % Bi 3+ , y mol % Eu 3+ : YV 0.5 P 0.5 O 4 under 266, 300, and 340 nm excitation normalized to the 5 D 0 → 7 F 1 transition of Eu 3+ . Inorganic Chemistry pubs.acs.org/IC Article emission is produced after an energy transfer whose efficacy is given in Table 2. Efficacy is 20% in the Bi 3+ -rich compounds F and G but tends to fall off in compounds containing more than 1% Eu 3+ : η is for instance comparable in compound D (3 mol % Bi 3+ , 1 mol % Eu 3+ ) and in compound I (10 mol % Bi 3+ , 2 mol % Eu 3+ ). Nevertheless, the emission intensity of compound J amounts to 6.5 times that of compound D, which demonstrates a synergy between Bi 3+ and Eu 3+ contents in YV 0.5 P 0.5 O 4 . Furthermore, with respect to both 355 and 397 nm excitations, compound J exhibits the highest emission intensity related to direct Eu 3+ excitation. At sufficiently low temperatures, vibronic interactions can be frozen out. Thus, the luminescence spectra can give more detailed information regarding the electronic transitions of Eu 3+ and can now be used as a structural probe. In 2008, Pan et. al 46 conducted investigations regarding the spectroscopic properties of Eu 3+ in Y(V,P)O 4 solid solution by laser-selective excitation. This work identified three symmetry sites in the yttrium orthovanadate-phosphate mixed compounds due to disorder generated by the distribution of (PO 4 ) and (VO 4 ) tetrahedra. The Judd−Ofelt intensity parameters further confirmed that significant changes in ligand polarizability contribute to differences in local environments experienced by Eu 3+ . 46 Figure 13 illustrates the excitation spectra collected at 5 K corresponding to the 619 nm emission which represents the Eu 3+ 5 D 0 → 7 F 2 transition. Low temperature excitation spectra depict noticeable differences when compared to the room temperature spectra. These major differences are as follows: (1) differences in relative intensity of the intrinsic 7 F 0 → 5 L 6 transitions of Eu 3+ to the CT excitation bands in compounds containing low amounts of Bi 3+ (<5 mol %). This indicates a less efficient sensitization at 5 K. Furthermore, it demonstrates that in these conditions, the  This suggests these transitions are not involved in the sensitization process of Eu 3+ at 5 K. Therefore, only the Bi−V MMCT operates as a sensitizing channel for Eu 3+ at 5 K. The emission spectra for 1, 3, 5, 10, 15 mol % Bi 3+ , 1 mol % Eu 3+ : YV 0.5 P 0.5 O 4 were collected at 5 K. These samples were excited in the CT bands and at 395 nm ( Figure 14). The wavelengths of the Stark components observed in Y 0.98 Eu 0.01 -Bi 0.01 V 0.5 P 0.5 O 4 and Y 0.89 Eu 0.01 Bi 0.10 V 0.5 P 0.5 O 4 are compiled in Table 3 and compared to reference data on Eu 3+ in YPO 4 and YVO 4 . 47 Data related to the 5 D 0 → 7 F 4 transition of Eu 3+ are incomplete, when compared to the reference zircon compounds YPO 4 :Eu 3+ and YVO 4 :Eu 3+ . The 5 D 0 → 7 F 1 transitions are also poorly resolved despite the low temperature. Thereby the results are discussed based on the 5 D 0 → 7 F 2 transitions. Two different signatures are noted for the Eu 3+ transitions in Y 0.98 Eu 0.01 Bi 0.01 -V 0.5 P 0.5 O 4 depending on the excitation wavelength. This indicates the presence of more than one Eu 3+ type site in this crystal structure, which agrees with the conclusions of Pan et al. 46 In Y 0.89 Eu 0.01 Bi 0.10 V 0.5 P 0.5 O 4 , however, only four Stark components (instead of five) corresponding to the 5 D 0 → 7 F 2 transition were observed. The spectrum in the 5 D 0 → 7 F 4 region looks also simpler. This simplification of the spectral signatures is consistent with the progressive crystal structure reordering of YP 0.5 V 0.5 O 4 concomitant with an increase in Bi 3+ concentration. This is previously observed in XRD results (Figure 2). Compared to YPO 4 :Eu 3+ and YVO 4 :Eu 3+ , the 5 D 0 → 7 F 2 spectrum of Y 0.89 Eu 0.01 Bi 0.10 V 0.5 P 0.5 O 4 is more split (265 cm −1 Figure 12. Integrated 5 D 0 → 7 F 2 emission intensity (•) and average emission lifetime of Eu 3+ ( □ ) for the different compounds listed in Table 2 in correspondence with λ exc = 397 nm (a) and λ exc = 355 nm (b).  Inorganic Chemistry pubs.acs.org/IC Article against 178−180 cm −1 ), and its energy barycenter is upshifted. This is due to the presence of the Bi 3+ ion in the second cationic neighborhood of Eu 3+ , as we have depicted in Figure 3. Owing to the BVS values given in Figure 1, it is concluded that the formal charge carried by bismuth is below that of yttrium, with the consequence that the formal charge carried by phosphorus and vanadium atoms in the Bi-doped compounds is enhanced with respect to the Bi-free compounds. This, in turn, affects the formal charge carried by the oxygen atoms in the first coordination of Eu 3+ by reinforcing the crystal field and softening the nephelauxetic effect.

CONCLUSION
The pure crystal phase of x mol % Bi 3+ , y mol % Eu 3+ : YV 0.5 P 0.5 O 4 was formed using a coprecipitation synthesis method. It is observed that codoping with Bi 3+ is followed by narrowing of the XRD peaks, but codoping with Eu 3+ does not significantly affect the widths of the peaks. The insufficient energy transfer to Eu 3+ , resulting from the Bi−V MMCT and the 3 T 1,2 → 1 A 1 (VO 4 ) 3− broad transition bands, is observed. Yet, Eu 3+ ion emission is enhanced by increasing Bi 3+ ion concentration. Further reaffirmed is the presence of more than one Eu 3+ site due to (PO 4 ) 3− and (VO 4 ) 3− substitution.
Additionally, an ordering of the crystal structure of YP 0.5 V 0.5 O 4 with increasing the Bi 3+ content can be observed. It is supposed that at low temperature the sensitizing pathway of Eu 3+ is less efficient in comparison to the room temperature, and mainly the Bi−V MMCT contribution is noticeable. Based on luminescent decay times, the Bi−Eu ET efficiency was found to be the highest for bismuth-rich compounds.